In recent years, a metal oxide having semiconductor characteristics called an oxide semiconductor has attracted attention as a novel semiconductor having high mobility provided by crystalline silicon and uniform element characteristics provided by amorphous silicon. The metal oxide is used for various applications. For example, indium oxide, which is a well-known metal oxide, is used for a light-transmitting pixel electrode in a liquid crystal display device, a light-emitting device, or the like. Examples of such a metal oxide having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. Transistors each including such a metal oxide having semiconductor characteristics in a channel formation region have been known (Patent Documents 1 and 2).